Dense breasts have less fatty tissue and more non-fatty tissue compared to breasts that aren't dense. Dense breasts have more gland tissue that makes and drains milk and supportive tissue (also called stroma) that surrounds the gland. Breast density can be inherited, so if your mother has dense breasts, it's likely you will, too.

Research has shown that dense breasts:

can be 6 times more likely to develop cancer

can make it harder for mammograms to detect breast cancer; breast cancers (which look white like breast gland tissue) are easier to see on a mammogram when they're surrounded by fatty tissue (which looks dark)

One way to measure breast density is the thickness of tissue on a mammogram.

The Breast Imaging Reporting and Database Systems, or BI-RADS, which reports the findings of mammograms, also includes an assessment of breast density. BI-RADS classifies breast density into four groups:

Mostly fatty: The breasts are made up of mostly fat and contain little fibrous and glandular tissue. This means the mammogram would likely show anything that was abnormal.

Scattered density: The breasts have quite a bit of fat, but there are a few areas of fibrous and glandular tissue.

Consistent density: The breasts have many areas of fibrous and glandular tissue that are evenly distributed through the breasts. This can make it hard to see small masses in the breast.

Extremely dense: The breasts have a lot of fibrous and glandular tissue. This may make it hard to see a cancer on a mammogram because the cancer can blend in with the normal tissue.

Still, no one method of measuring breast density has been agreed upon by doctors. Breast density is not based on how your breasts feel during your self-exam or your doctor's physical exam.

A study that surveyed women living in Virginia suggests that many women don’t know that having dense breasts increases their risk of breast cancer and makes it harder for mammograms to detect cancer.

About 43% of women ages 40 to 74 years old in the United States are considered to have dense breasts.

As of May 2016, 27 states have passed laws requiring that women be notified of their breast density with mammography results. Virginia is one of these states.

To do the study, the researchers contacted a random sample of 1,024 Virginia women by landline or cell phone. The women were asked questions about breast density and breast cancer risk. The phone survey took about 24 minutes to complete.

The researchers found:

36% of the women had been informed of their breast density

less than 1% of the women said breast density was a risk factor for breast cancer

about 50% of the women who had had a mammogram in the last year were aware of their breast density

Overall, only 1 in 5 women were aware that having dense breasts makes it hard for a mammogram to detect breast cancer. Only 1in 8 women were aware that having dense breasts increases breast cancer risk.

The most important factor in whether a woman knew about breast density and its link to higher risk was whether a doctor or nurse had talked to her about her breast density. The researchers emphasized that it was very important for women and their healthcare providers to talk about breast density.

"The most important thing that doctors and patients can take away from this study is that the required written notice about breast density isn't enough in itself: patients need to talk with their providers about what breast density means for each woman's individual breast cancer risk," said Thomas Guterbock, University of Virginia professor of sociology and director of the university’s Center for Survey Research who was lead author of the study.

As this study strongly suggests, it’s a good idea to talk to your doctor about your breast density each time you receive your mammogram results. If your mammogram reports consistently say that you don’t have dense breasts or that you do have dense breasts, then that is likely to be accurate. Still, if your breast density varies from mammogram to mammogram, talk to your doctor or the radiologist who read the mammogram to get a better understanding of your breast density and how it affects your risk of breast cancer.

For more information on breast density and steps you can take to keep your breast cancer risk as low as it can be if you do have dense breasts, visit the Having Dense Breasts pages in the Breastcancer.org Lower Your Risk section.